The Mac version of Revenge of the Titans, as delivered through Steam, crashes randomly after a few minutes. The Puppygames version doesn't, despite being identical. (If you got Revenge of the Titans via the Humble Indie Bundle #2, use that version instead)

It turns out the Steam client is actually the bit that is crashing.

Some people have had success with preventing the problem by turning off Steam Community Integration and turning off Keep this game up-to-date. YMMV.

That'll be something completely different - can you send me your err.log file (in ~/.revenge_of_the_titans_1.80), and any output from the System Console when starting the game. Some machine specs would be handy too.